Is the grandfather's sale of the house to his son in exchange for the son's payment of the grandfather's debt and the grandfather's marriage of his other children valid according to Sharia, and does this sale prevent the other children from claiming inheritance in this house after the grandfather's death?

If the father repaid his father’s debt without intending to seek reimbursement or claim it back, and then the grandfather insisted on giving him the house in return, then there is a detailed explanation:

1. If the grandfather did not accept the son's voluntary repayment of the debt from the outset and insisted on compensating him, then the gift is considered invalid due to the lack of acceptance.

2. If the grandfather accepted the repayment of the debt as a donation, and then wished to compensate him, then the son's donation is valid, and he will be rewarded for it. This compensation is considered a gift, and the principle regarding gifts to children is justice among them, unless the rest of the adult, mature children agree, or if the preference is for a legitimate reason according to Sharia. In this case, the preference for your father is not a mere preference; rather, it is for a legitimate reason, which is what he paid to settle the debt, and because he did not receive anything for his marriage. Therefore, there is no harm in the grandfather giving the house to your father, as long as its value is less than the debt your father repaid.